A secondary battery is conventionally charged using a method in which a constant current charge is conducted until a terminal voltage of the secondary battery reaches a predetermined voltage value, and then switched to a constant voltage charge after a predetermined voltage value has been reached. According to the above method, during the constant voltage charge after the predetermined voltage value has been reached, the secondary battery is determined to be fully charged when a predetermined time has elapsed after the moment a charge current at predetermined current value (current threshold value) or less is detected.
According to the above-described method for determining a full charge, if a secondary battery that is fully or nearly fully charged is charged, a charge current being the threshold value or lower is detected immediately after charging is started and, after a predetermined time has elapsed since that moment, the secondary battery is determined to be fully charged. However, the problem arises that, regardless of the battery already being fully charged before the elapse of the predetermined time, the full charge cannot be determined until the predetermined time elapses. Hence, according to Patent Document 1, the full charge is determined before the elapse of the predetermined time when the initial charge current is a current threshold value or less by switching the method for determining a full charge according to an initial determination result of the charge current value after charging is started.
Further, Patent Document 2 discloses a method for detecting a state of charge at a charge acceptance limit, which is an upper limit of the state of charge chargeable to the secondary battery. Here, the prior art determines if conditions are satisfied, the conditions being a measured temperature value higher than a predetermined temperature threshold value, a measured voltage value higher than a voltage threshold value, an average current within a predetermined current threshold value range, and a voltage exceeding rate higher than a predetermined voltage exceeding rate threshold value. Then, when the method determines that the conditions are satisfied, a condition satisfaction duration time over which the conditions are continually satisfied is calculated and, when the condition satisfaction duration time is determined to be longer than a predetermined duration time threshold value, the state of charge at that time is regarded as a state of charge at the charge acceptance limit.